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The vent needs to be before the trap. Yes, the unit will suck air though the trap during operation instead of sucking \ holding the water in the trap. The vent can be screened for sanitary concerns.

Vent prior to trap would negate the trap. I worked with a guy years ago that said he used swing check valves, never did it myself so I have no idea if it would work.

Originally Posted by NWAIR

I have always installed a riser vent prior to the trap to alleviate this problem.

I think the vent needs to be after the trap, if it's before the trap, the unit will suck air through the vent.

I have always installed a riser vent prior to the trap to alleviate this problem.

1" minimum between the bottom of the 1st elbow out of the pan and the top of the last elbow out of the trap into the run out line. 2" is better on larger style Trane units where the unit is pulling the air thru the coils

Originally Posted by timebuilder

Install a new trap.

The important part is the depth of the outlet leg. The inlet leg needs to be just a little longer, so the inlet fills the entire trap and the outlet is maybe 1/4 inch below the inlet level.

About 2 1/2 inches deep for the oulet will ensure the water can get out.

Thanks for all the info guys.

Yes it's defiantly the trap.. Had a guy come in and he agreed the trap should have been set up better but the installer screwed them selves since the unit wasn't high enough off the ground.. Also we noticed the pitch off the outlet was not correct either so he adjusted that and seems to be ok for now. I think we have to put a secondary pan in the bottom of the unit to ensure this doesn't happen again.

4 techs couldn't figure this out or didn't want to! Un real!

Dirty evaporator would also make it hard to open a cover.

Install a new trap.

The important part is the depth of the outlet leg. The inlet leg needs to be just a little longer, so the inlet fills the entire trap and the outlet is maybe 1/4 inch below the inlet level.

About 2 1/2 inches deep for the oulet will ensure the water can get out.

I had the same problem at a restraunt recently. The drains were trapped and tied into a branch drain. All were clear and had good slope. However, about once a week they would back up at random. After spening way too much time looking at these ah, I figured there had to be something else causing these lines to develop a vac. I did some checking and found that the make up air was non opp. After the hoods would run for a while, they would pull the building into a neg pressure situation and pull air back in from the branch drains.

Originally Posted by Six

Usually when you have that much negative suction its a problem with insufficient return. Ive tapped into return plenums and added flex to new filter grilles to fix that in the past.

It is located in a tiny closet, you can only access one half of the unit. The dampers on the return face a wall and have about a foot of space on front of it that is in addition to the normal return vent would that effect it? Seems to very poorly installed. I'm actually finding trash in the unit! So I think when the dampers open it sucks in what ever is in that room and some how gets passed the filters.

Usually when you have that much negative suction its a problem with insufficient return. Ive tapped into return plenums and added flex to new filter grilles to fix that in the past.

Trap needs fixed. having the inlet and outlet the same height doesn't work real well on most units. Also, return is probably not correct. might want to try a lower PD air filter, if more return can't be added.

Originally Posted by Matt_M

I had a problem a 15 ton ahu no draining, on a new install. installers put like a 3 inch deep trap on a 1 inch pvc line. Manual called for min of 6inch. Installed correct trap size, with the outlet noticeably lower than inlet, problem solved. Check product manuals, if available and go from there. If none are available, i would increase trap depth and drop the trap outlet lower than inlet, if you can. Do you have enough R/A going to unit? Having a hard time opening a cover tells me your a little light in R/A.

I will have to check for a manual.. The tricky part is there really isn't enough space to drop the trap any more considering how low to the ground the drain is and the length of the drain line run to the floor drain. Would a pump be a option after the trap or even with out the trap?

The sad part is the customer had a decent local company come out 4 times with 4 different techs and they were saying different things. Basically at the end they said they would have to replace the drip pan inside because it was rusty and the rust was blocking the line. Well most the rust is just surface rust and I didn't see any chunky stuff so that is a BS answer that they were going to charge 16 hours of labor to fix. I own a facilities maintenance company and my guys will do some PM work for our customers and that's usually it. So for me to go in and know its not a rust issue and to pretty much figure out what the issue is doesn't say a lot for that service company. Their company installed the unit so I'm sure they don't want to confess to installing the trap wrong.

This caused severe water damage twice this year to a business next door and it needed to fixed ASAP and they came out agin Friday blow out the drain again and next day it was flooding again. Funny when in after that checked the drain not a spec of anything in it. They told the customer when they called on Sat they wouldn't return till next week even though it was flooding again! Awesome service!

Moved to Tech to Tech commercial forum.

I had a problem a 15 ton ahu no draining, on a new install. installers put like a 3 inch deep trap on a 1 inch pvc line. Manual called for min of 6inch. Installed correct trap size, with the outlet noticeably lower than inlet, problem solved. Check product manuals, if available and go from there. If none are available, i would increase trap depth and drop the trap outlet lower than inlet, if you can. Do you have enough R/A going to unit? Having a hard time opening a cover tells me your a little light in R/A.

Originally Posted by Matt_M

Is the trap deep enough and primed with water? You could be pulling water back into the unit if the blower is big enough and the trap is too shallow.

It is primed but you may be right the may not be deep enough. It's a negative air system but the pipes are pretty much even with each other where it enters the trap the leaves the trap. My understanding is the side where it leaves the trap should be considerably lower then where it enters. Correct?

It's a fairly large unit.. You can barely pry the access cover off when unit is running.

Is the trap deep enough and primed with water? You could be pulling water back into the unit if the blower is big enough and the trap is too shallow.

Condensate line issues.

I'm having trouble figuring out why there is a vaccum effect on a condensate line on a unit at a restaurant we work for. With everything closed up you can feel a slight suck at the end of the pipe where it drains out. The only way it stops is if you leave the access cover off the unit. This has been causing the unit to not drain properly and back up and flood the area. There is a trap in the drain line. Not really sure what else it could be.

Thanksfor the help!